How accurate is python's time.sleep()?

Question

I can give it floating point numbers, such as

time.sleep(0.5)

but how accurate is it? If i give it

time.sleep(0.05)

will it really sleep about 50 ms?

Answer 1

From the documentation:

On the other hand, the precision of time() and sleep() is better than their Unix equivalents: times are expressed as floating point numbers, time() returns the most accurate time available (using Unix gettimeofday where available), and sleep() will accept a time with a nonzero fraction (Unix select is used to implement this, where available).

And more specifically w.r.t. sleep():

Suspend execution for the given number of seconds. The argument may be a floating point number to indicate a more precise sleep time. The actual suspension time may be less than that requested because any caught signal will terminate the sleep() following execution of that signal’s catching routine. Also, the suspension time may be longer than requested by an arbitrary amount because of the scheduling of other activity in the system.

Answer 2

The accuracy of the time.sleep function depends on the accuracy of your underlying OS's sleep accuracy. For non-realtime OS's like a stock Linux Kernel or Windows the smallest interval you can sleep for is about 10-13ms. I have seen accurate sleeps within several milliseconds of that time when above the minimum 10-13ms.

Update: Like mentioned in the docs sited below, it's common to do the sleep in a loop that will make sure to go back to sleep if you have woken up the early.

I should also mention that if you are running Ubuntu you can try out a pseudo real-time kernel by install the rt kernel package.

Answer 3

You can't really guarantee anything about sleep(), except that it will at least make a best effort to sleep as long as you told it (signals can kill your sleep before the time is up, and lots more things can make it run long). For sure the minimum you can get on a standard desktop operating system is going to be around 16ms (timer granularity plus time to context switch), but chances are that the % deviation from the provided argument is going to be significant when you're trying to sleep for 10s of milliseconds. Signals, other threads holding the GIL, kernel scheduling fun, processor speed stepping, etc. can all play havoc with the duration your thread/process actually sleeps.

Answer 4

Why don't you find out:

from datetime import datetime
import time

def check_sleep(amount):
    start = datetime.now()
    time.sleep(amount)
    end = datetime.now()
    delta = end-start
    return delta.seconds + delta.microseconds/1000000.

error = sum(abs(check_sleep(0.050)-0.050) for i in xrange(100))*10
print "Average error is %0.2fms" % error

For the record, I get around 0.1ms error on my HTPC and 2ms on my laptop, both linux machines.